The present invention relates to a method and apparatus for recognizing the cutoff state of a thyristor that can be switched off (GTO thyristor), especially also after an unsuccessful attempt to switch it off if the thyristor is alloyed-through (shorted), by forming after the occurrence of a "turn-off" signal, a state signal indicating the advent of the cutoff state from the derivitive in time of the gate current.
Such a method is preferably applied in converter circuits with GTO thyristors (gate turnoff thyristors), for instance, in bridge arrangements in which the mutual relief of the thyristors during conduction is to take place in optimum time during the current commutation from one bridge arm half to the other. For preventing a short circuit, pauses must be observed between the individual thyristor firing signals so that even for a short transition time, both thyristors of a bridge arm never carry current at the same time. These pauses, however, depend not only on the prevailing operating state of the thyristor, characterized by its actual barrier layer temperature, the forward blocking voltage and the forward current, but also on its external wiring which is necessary for safe operation and which preferably serves for facilitating the switching. The value of the minimum permissible pause period is basically obtained from the worst operating point with maximum thyristor quenching time, possibly taking the time behavior of the additional switching relief devices of their own into consideration. However, along with this goes an overall non-optimum operating behavior of the converter. If, on the other hand, the method according to the present invention is used, these pauses are reduced, by an adaptive mutual interlock of the thyristor "on" signals, to a minimum value which is necessary for the prevailing operating state of the thyristors and their external wiring. The relief in the conduction can now take place in optimum time since the instant of the advent of the blocking switching state of the thyristor to be relieved is actually recognized and need not be estimated by means of a so-called worst-case consideration.
Especially also in the event of a defect when permanent conductance between the anode and the cathode of the GTO thyristor due to the alloying-through of the semiconductor layers occurs, protective measures can be initiated in time by the application of the method according to the invention if the state signal characterizing the cutoff state fails to arrive. Besides triggering trouble messages, it is particularly important here to suppress the "on" signal for a further still intact thyristor in the arm of a bridge circuit for preventing a short circuit.
In German Patent Application No. P 34 34 607.4, three methods for recognizing the current-blocking switching state have been described for the operation of GTO thyristors.
Thus, in the first of the proposed methods, after the occurrence of an "off" command, the negative voltage between the gate and the cathode of the thyristor is determined and a state signal characterizing the advent of the cutoff state is formed if the gate-cathode voltage falls below a negative threshold.
A second method determines, instead of the gate cathode voltage at the thyristor, the negative control current flowing in its gate terminal during the switching off process and forms the state signal characterizing the advent of a blocking state if the derivative in time of the controlling gate current changes its sign after the occurrence of an "off" command.
In a third method, the state signal is formed in a further development of the second method, if either the derivative in time of the gate current changes its sign and the gate current itself has fallen below a negative threshold or the derivative in time of the gate current, while it has changed its sign, has not fallen below the negative threshold within a predetermined time.
With the aid of the internal behavior of the control circuit during the switching-off process, for instance, by means of the voltage drop caused by the negative control current at the output of the addressing unit, in the following called the driver, due to an output impedance different from zero, conclusions can be drawn as to the instantaneously flowing gate current and its derivative in time, so that the actual switching state and, in particular, the cutoff state, can be recognized.
The necessary minimizing of the actual undesired voltage drop at the output of the driver, however, requires overall a relatively small signal-to-noise margin of the signals to be evaluated. In addition, variations of the amplitude of the negative gate current which depends on the magnitude of the actual anode-cathode current of the thyristor can lead to a further reduction of the already small voltage drop. These circumstances can make the timely recognition of a permanent short circuit within the thyristor due to alloying-through of the semiconductor layers more difficult in some cases, for instance, if after a "turn-off" command, the cutoff state actually does not occur.